The text of U.S. application Ser. No. 08/428,332 is incorporated by reference as if set forth fully herein.
This invention relates generally carburetors for internal combustion engines and more specifically to a metering device of a carburetor.
High performance as well as fuel economy of an internal combustion engine, and in particular automobile racing engines, is greatly affected by operation of the engine's carburetor. In general, the carburetor must feed the proper amount of fuel to air stream flowing into the engine, at the proper time, so that the engine runs smoothly and with the power needed, without wasting fuel or running too hot. The proper amount of fuel to be metered to the air stream will depend on several factors, including the construction of the particular engine and ambient air conditions. Generally speaking when the ambient air is cool and dry, its density increases so that more fuel is required to maintain the proper mass ratio of fuel to air in the cylinders. In contrast, warm humid ambient air conditions reduce the density of the air in the cylinder so that less fuel should be supplied. Thus, to achieve the optimum operation of the carburetor under different conditions, it is desirable to adjust the carburetor.
A typical carburetor includes a main body through which a stream of air from the air intake passes to the manifold, and in which the fuel is fed to the air stream. A fuel bowl holding a reservoir of gasoline to be fed to the air stream is mounted on the main body by a metering block through which a measured flow of fuel is aspirated from the fuel bowl to the air stream in the main body. One face of the metering block forms a wall of the fuel bowl which is usually immersed about half way up the face in the gasoline in the bowl. The metering block has an idle/transfer circuit which transfers fuel to the main body when the engine is running at low to moderate speeds, and a main circuit which transfers fuel at higher engine speeds. Some of the air received in the main body is diverted to a main air bleed in the metering block and thence to the main fuel and idle/transfer circuits for premixing with the liquid fuel from the fuel bowl prior to the introduction of the fuel to the air stream in the main body. The introduction of air facilitates the atomization of the fuel needed to produce a combustible air/fuel mixture for the engine.
Optimum performance of the engine depends in part upon the degree of atomization of the fuel which reaches the combustion chamber of the engine. The greater the mixing of air and fuel, the higher the percentage of fuel which is burned in the combustion chamber. The atomization produced by the main air bleed in the metering block and by aspiration of the liquid fuel into the air flow through the main body of carburetor, while good, still delivers a substantial amount of fuel to the combustion chamber in drops which are too large to burn.
Presently, main fuel circuits and idle/transfer circuits in metering blocks have several changes in direction within the metering block. Frequent and/or sharp changes in direction cause the fuel to flow turbulently through the metering block. It requires more energy and is more difficult to move the fuel through a tortuous path. In addition, these circuits tend to be somewhat long, causing small but measurable delays from the time when the throttle is open and the engine calls for additional fuel, and the actual delivery of fuel to the air stream in the main body.
The amount of fuel drawn from the fuel bowl depends upon the momentum of the air stream flowing through the main body and on the size of the restriction in the idle/transfer circuit and main circuit in the metering block. The restriction is achieved by insertion of a metering jet into the circuit in the metering block. Most commonly, the metering jets for the main circuit are placed in the intake port for the main circuit on the first face of the metering block in the fuel bowl. The metering jets can be changed, but require disassembly of the fuel bowl and metering block from the main body to do so. Thus, for example, it would be completely impractical to adjust the metering of the main circuit during the course of a race, and very difficult to make rapid alterations when testing an engine on a dynamometer.
There are presently existing metering blocks which permit an adjustment of the air/fuel ratio in the air stream in the carburetor without disassembly of the fuel bowl and metering block from the main body. The Total Control Metering System metering block made by Racing Engine Components of Colorado Springs, Colo. has a single air/fuel ratio adjuster which can be manipulated while the metering block and fuel bowl are mounted on the main body. The air/fuel ratio adjuster includes metering portions which are disposed in both main fuel circuits of the metering block downstream of the location where air is introduced to the fuel from the main air bleed. A wide range of flow rates is permitted. An actuator portion of the adjuster extends from the metering portion to a location outside an end wall of the metering block where it is accessible for adjustment without disassembly of the carburetor. The metering portion is shaped so that by turning the adjuster on its longitudinal axis within the metering block, the effective cross sectional areas of both main fuel circuits are changed in the same way to change the fuel/air ratio.
Changing both main fuel supply circuits in the metering blocks limits the precision of the adjustment. In many cases, the optimal allocation to each cylinder of fuel fed through a single metering block is not the same for each cylinder. In addition, permitting a wide range of change in the flow rates of fuel increases the chance that an engine might be damaged as a result of an improper fuel flow setting for a particular engine under certain ambient conditions.
Adjusting the fuel flow to the main body through use of an air/fuel ratio adjuster which meters the flow of an air/fuel mixture has certain problems. Substantially more fuel enters the metering block and flows past the location where air is introduced to the fuel than will ultimately be metered to the air stream in the carburetor. Thus, less air is mixed with the fuel in the metering block. The flow of the air/fuel mixture through the restriction caused by the metering portion of the adjuster tends to become irregular. Moreover, the restriction tends to separate the fuel from the air so that the advantage of pre-mixing the fuel with air in the metering device prior to introduction into the air stream in the main body, is lost to a significant degree. Thus, it is possible for the fuel to be insufficiently atomized when it enters the air stream, adversely affecting combustion in the engine.